In power storage elements and electric double layer capacitors, activated carbon with a large specific surface area has been used as polarizable electrodes. In use of an aqueous solution as an electrolytic solution, there was a problem that the usable withstand voltage is at most about 1 V, which is the electrolysis voltage of water. When using an organic electrolytic solution as an electrolytic solution, it is possible to use a withstand voltage of 3 V or more depending on the kind of the organic electrolytic solution employed. In such cases, therefore, it becomes possible to increase the electrostatic energy, which is proportional to the square of voltage, with increase in withstand voltage.
Moreover, an electric double layer capacitor with an electrostatic capacitance greater than those of electric double layer capacitors using conventional activated carbon has been proposed to be provided by using, as a carbon material for constituting a polarizable electrode, a carbon material being produced by performing a heat treatment together with at least one of an alkali metal and an alkali metal compound, the heat treatment being conducted at a temperature which is not less than a temperature at which a vapor of the alkali metal is generated; and developing an electrostatic capacitance through application of a voltage of not less than a rated voltage firstly between the polarizable electrodes after assembling the electric double layer capacitor so that ions of a solute of the organic electrolytic solution are inserted into a space between microcrystalline carbon layers composed of the carbon material (see, for example, Patent Document 1). Patent Document 1: Japanese Patent Laid-Open Publication No. 2000-77273